(NEW YORK) — Nine firefighters are injured, including two who are in critical condition, following an explosion in Los Angeles, according to the LA Fire Department.

The blast occurred six minutes after 10 firefighters arrived on scene responding to a semi-truck on fire early Thursday, the department said.

The semi-truck does “not run on diesel or on gasoline, but it is actually propelled … with compressed natural gas,” Fire Department Capt. Erik Scott said at a news conference.

Two 100-gallon tanks were on the truck, one of which exploded while firefighters were putting out the blaze, Scott said.

The flames were as high as telephone poles and exploded a transformer nearby, he said.

Two firefighters were critically hurt, four suffered moderate injuries and three had minor injuries, Scott said.

“One of those firefighters received specialized care at the burn center and he has already been airlifted to LA General Hospital for further care,” LA Fire Chief Kristin Crowley said.

No civilians were hurt, including the truck driver, Scott said.

Critical care surgery specialist Dr. Molly Deane said she saw video of the explosion.

“It’s remarkable that none of them were more severely injured after watching the footage,” she said.

There’s no continuing threat to the public, Crowley said.

“We did have flames that continued from one of those cylinders, the non-exploded one, for several hours,” Scott said. “We then established a 500-foot perimeter around that blast zone to ensure safety.”

“We still have firefighters on scene, along with our friends from law enforcement, that are keeping the public outside that perimeter, and this does remain an active incident,” he said.

“While Angelenos were barely waking up and making their first cup of coffee, our LAFD firefighters were courageously responding to this blaze, putting their lives on the line to protect each one of us, as they do every single day,” LA Mayor Karen Bass told reporters. “I want to acknowledge the families of the firefighters who were injured this morning, and all of their firefighter colleagues: Our thoughts are with you as we all hope for a rapid recovery for all involved.”

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